The process heats oil shale in a sealed horizontal segmented vessel (retort) causing its decomposition into shale oil, oil shale gas and spent residue.
The related cancer, mule spinners' carcinoma, was blamed on the carcinogenic content of shale oil that was used to lubricate the rapidly revolving mule spindles.
According to this, production of a barrel of shale oil can generate up to 1.5 tonnes of semi-coke, which may occupy up to 25% greater volume than the original shale.
Onshore contract drilling companies are taking advantage of the investments by principal companies to exploit unconventional shale gas, shale oil and coal bed methane reservoirs.